I see it as being much closer to the concept of signed, numbered copies of a lithograph, say an M. C. Escher work. Any pirate can xerox one of those, but knowing you have your "official" copy is more about accompanying documentation than the object itself. In this case, the documentation is a Serial Number. The easily copied object is the collection of data files. If it is legal and feasible for numbered artwork in a printed medium, why isn't it legal and feasible for numbered artwork in a digital medium?